1. Field of the Invention
The present invention relates to an ink jet printer, and particularly to an ink jet printer which applies pulse voltage based on image data to drive a piezoelectric element.
2. Description of the Related Art
This application is based on application No. 9-106121 filed in Japan, the contents of which is hereby incorporated by reference.
An ink jet printer using a piezoelectric element in its head is known. Pulse voltage according to image information is applied to the piezoelectric element of the head of such an ink jet printer. Deformation of the piezoelectric element caused by the application of the pulse voltage pressurizes ink in a prescribed container (ink channel), and ink drops are emitted from a nozzle provided to the ink channel toward a recording sheet. The ink drops fly to record an image on the recording sheet.
A demand for a printer adapted for full-color printing is growing due to an improving network environment and prevalence of such a device as a digital camera. In order to satisfy such a demand, a technique for enhancing the quality of a printed image by using such an ink jet printer is developing. A technique of increasing levels of gradation of an image is required for outputting a high quality image by the ink jet printer.
As a method for reproducing gradation by an ink jet printer, a method of changing an area of a dot produced by impact of a single ink drop on a recording sheet is known. By this method, a degree of deformation of a piezoelectric element in a head is controlled, that is, the amplitude (maximum value) of pulse voltage applied to the piezoelectric element is changed to fly ink drops of different volumes from the same ink channel and the same nozzle.
A problem of the method of changing only the maximum value of the pulse voltage is that a dynamic range (a range of a diameter of a dot which can be output by the same nozzle) is limited by the material property of ink such as the mass and viscosity of the ink, a diameter of a nozzle, a structure of an ink channel, and the like. Therefore, it is very difficult to increase the dynamic range to improve a quality of an image. Further, increase of frequency that drives a piezoelectric element is highly difficult due to such limitation.